1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method, and specifically to an image processing apparatus for performing correction of density values of image data, correspondingly to each of a plurality of printing elements for a printing apparatus or each of a plurality of rasters constituting the image data corresponding to those printing elements.
2. Description of the Related Art
Conventionally, a printing apparatus for performing printing on printing media, such as papers, OHP (Over Head Projector) sheets, etc. (hereinafter referred to as “printing paper” or simply “paper”) employ several printing methods. As apparatuses of such printing methods, the printing apparatus of the wire dot printing, the thermal sensitive printing, the thermal ink-transfer printing, the ink-jet printing and the like are known. Among those apparatuses, the printing apparatus of the ink-jet method ejects ink directly onto the printing paper to perform printing and are used widely as an image output apparatus, such as a printer, a facsimile, a copying machine, etc, because the apparatus of the ink-jet method has various merits such as low noise, a low running cost, easiness of miniaturization and color printing, etc.
It is known that in printing apparatuses of the various methods stated above, there is a problem of density non-uniformity. For example, a print head for the printing apparatus of the ink-jet method generally comprises a plurality of ink ejection orifices (hereinafter also referred to as “nozzle”) arranged in a direction of conveyance of a printing medium and through the plurality of ejection orifices ejects an ink droplet while the printing head scans the printing medium, so that printing is performed. In such print head having the plurality of nozzles, differences in an ejection characteristic among the nozzles may be caused due to a slight error in manufacturing the print head and due to aging of the print head. This difference may cause the density non-uniformity such as a black streak, a white streak, or the like in a printed image.
To solve this problem, for example, Japanese Patent Application Laid-open No. 5-220977 (1993) discloses a so-called head shading method. In the method density correction data is prepared for each nozzle and image data is corrected with respect to raster data for each nozzle by means of respective the density correction data so that the density non-uniformity in the printed image can be prevented from occurring.
However, the conventional head shading method is one where the density correction is performed by relating one piece of the density correction data to each printing element. Therefor, in the ink-jet printing apparatus where various printing conditions are used to perform printing, for example, plural kinds of printing modes are used for printing, there may be a case where, depending upon the printing condition, correction data for the density non-uniformity is not suited to the printing condition and hence the density non-uniformity correction based on that correction data may not be properly reflected on the printed image.
For example, in the case of differentiating the printing condition by differentiating kinds of images to be printed, when an image to be printed is a photographic-like pictorial image, most of the image to be printed is a halftone image. Therefore, the variation in the printing characteristic among printing elements may come out as the density non-uniformity in the printed image in a relatively noticeable manner, hence degrading quality of the printed image. On the other hand, when an image to be printed is one that consists of mainly characters and graphics, a portion of a halftone may not exist so much but portions of the so-called solid image (images without a halftone portion) may be most. Therefore, even if the variation in the printing characteristic of the printing elements exist, there may be little influence on the quality of the image. This is because an area factor defined as a covering ratio of a printed dot to a printing paper is intrinsically more than 100 percent for the solid image. Therefor, even if there exists variation in for example ejection quantity among the nozzles serving as printing elements and consequently variation in the dot size formed on the printing paper, it scarcely appears as the density non-uniformity. Contrary to this, in the case of the halftone image, since the above-mentioned area factor is relatively small, if there exists variation in the size of the formed dot, it is likely to appear as variation in the density.
In such cases, it is probable that the correction data determined for each printing element is set in such a way that the correction data is intended to be suited to either one kind of images, or strictly speaking, the correction data is suited to neither kind of images. As a result, it may be a case that correcting of the density non-uniformity using the correction data cannot cause a proper reflection on the printed image, and consequently low-quality printing with the density non-uniformity being remained unsolved is performed. When the correction data is produced, a test pattern is printed. For example, if the test pattern consists of several patches each covering plural levels of the possible gray level, the correction data produced based on these test pattern is what is suited to the halftone image more likely than not.
Moreover, if the correction data is set so that the density correction for the halftone image is performed properly, a density value is overcorrected to an excessively low value and hence the printing dot size becomes extremely small to effect decrease in the area factor when printing the solid image, and conversely the density non-uniformity may occur.
It is needless to say that the printing condition in which the variation in the characteristic of printing elements noticeably comes out as the density non-uniformity is not limited to the above-described conditions. For example, in the ink-jet method, when there exists the variation in ejection direction of the nozzles other than the ejection quantity, it is often the case where the user recognizes the density non-uniformity called a white streak or black streak that is generated by the variation in formation position of the printed dot. In this case, this density non-uniformity doesn't necessarily appear in a noticeably different manner for the halftone image and for the solid image.
Moreover, being in conjunction of this case, not only in the case where the kinds of the image data to be printed are different as described above, but also in the following cases the density appears differently according to each condition; the case where the printing modes, such as a driving frequency of the printing elements when the printing is performed, are different; the case where scan directions of the printing head are different; and the case where the ejection quantity is modified for each nozzle acting as a printing element or the ink density is modified. Also in such cases, if each case has such a configuration similarly that one piece of the density correction data is provided for each printing element or each raster, the density correction may have no proper reflection on the printed image.